Semiconductor device and method of fabrication

ABSTRACT

A process for the fabrication of an integrated circuit assembly, using thin film platinum metallization to provide edge-side contacts suitable for solder ball connections. Three-dimensional laser ablation may be used for patterning metal films. A multi-chip assembly may be formed using orthogonal edge-side mounting on a substrate.

This invention relates to the fabrication and assembly of semiconductorchips, substrates, and modules, and more particularly to methods andapparatus for achieving flexible, low-cost manufacturing. Commercial andmilitary systems today are placing increasing demands on flexibleapplication and reliable operation, as well as on simplifiedmanufacturing.

BACKGROUND OF THE INVENTION

Semiconductor devices have been prepared in the past using variouscombinations of metallization over the circuit metal contact pads. Suchcombinations generally have included the use of thin films of refractorymetals as well as gold and palladium, which require costly patterningsteps involving hazardous material and chemical waste, and also generatestress in the semiconductor chips. They furthermore severely limit thechoice of soldering materials for assembly and packaging. Althoughplatinum, with titanium as undermetal, has been proposed in 1996(“Self-Aligned, Fluxless Flip-Chip Bonding Technology for PhotonicDevices”, by J. F. Kuhmann, H.-J. Hensel, D. Pech, P. Harde, and H.-G.Bach, Proc. 1996 Electronic Components and Technology Conference, May1996), it was restricted to specialty III-V photonic devices andeutectic solder connections. For rerouting processes on semiconductorcircuits, past technology offers only methods of questionableeffectiveness to position the thin metal films across the sides or edgesof the chip. In addition, the patterning processes for these thin metalfilms are expensive and generate liquid waste byproducts which must bedisposed of. For forming cubes from a plurality of stacked chips, theknown technology must rely on cumbersome, expensive methods. Knownmethods for assembling individual chips or finished cubes ontosubstrates often involve poorly compatible metallization and solders,with difficulty for aligning active parts and substrate.

In summary, the goal of offering for commercial and military systemscost-effective, reliable, rerouted semiconductor products, manufacturedin high volume and with flexible, low-cost production methods, hasremained elusive, until now.

SUMMARY OF THE INVENTION

In accordance with the present invention, there is provided first aplurality of silicon semiconductor devices for application in digitalsignal processing, microprocessor, memory and other commercial andmilitary applications requiring flexibility, high reliability andcost-effectiveness; secondly a process aiming at low-costmanufacturabilty, far reduced number of process steps and easy rework,all of which offer an economic advantage over the prior art and alsoavoid the generation of chemical waste byproducts which would requirecostly disposition; and thirdly, apparatus is provided for improvingselected steps of the process.

It is an object of the present invention to provide a low-cost methodand system for rerouting the circuit metal contact pads.

Another object of the present invention is to provide a method for wideassembly flexibility by using reroute metal and solder materialcombinations allowing a wide range of assembly temperatures.

Another object of the present invention is to simplify system rework byemploying simultaneously various solder/material combinations.

Another object of the present invention is to provide a method forpatterning the reroute metal so that it can operate cost-effectively intwo as well as in three dimensions.

Another object of the present invention is to provide a technology forcovering the small sides or edges of the chips in preparation forreliable extension of the reroute network across said small sides.

Another object of the present invention is to provide a method andapparatus for cost-effectively laminating semiconductor chips in athree-dimensional configuration to allow high density packaging.

Another object of the present invention is to provide a plurality ofconductive means on the interconnect substrate compatible with the chipconductors and solder material choices.

Another object of the present invention is to develop a flexible,efficient, economical, mass producible technology for dense packaging ofsemiconductor chips.

These objects have been achieved by a mass-production process using acombination of thin film platinum metallization, protective dielectricmasking, and three-dimensional laser ablation, in conjunction with avariety of solder combinations and melting temperatures. Thesecombinations have been employed for the fabrication of silicon chips aswell as connective substrates. Furthermore, spacing films with adhesiveproperties on both surfaces have been successfully used for assemblingmulti-chip cubes.

Other objects and features of the invention will become more readilyunderstood from the following detailed description and appended claimswhen read in conjunction with the accompanying drawings, in which likereference numerals designate like parts throughout the figures thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a cross section of a solder bump and undermetalarrangement over the chip contact metallization, according to knowntechnology.

FIGS. 2 to 5 illustrate the process flow for rerouting conductive pathsto circuit contact pads, according to the invention. FIGS. designated by“a” show bird's eye views, and FIGS. designated by “b” show crosssectional views.

FIG. 6 illustrates a more detailed cross section through part of asemiconductor chip after deposition and patterning of the reroutingplatinum and refractory metal layers.

FIG. 7 illustrates a metal contact pad and a remote part of thererouting after solder balls have been attached.

FIG. 8 illustrates the metallization on the substrate with thefootprints for the connection to a semiconductor chip and a passiveelement.

FIG. 9 illustrates the detail of a cross section through one contactbetween chip and substrate after reflow of the solder bump.

FIG. 10 illustrates the orthogonal assembly after solder reflow of achip with metal rerouting onto a substrate with conductor pattern.

FIGS. 11a and 11 b illustrate the fixture in cross section and top viewrespectively which allows the deposition of dielectrics over the sideedges of the chip.

FIGS. 12a and 12 b illustrate the same fixture with a modified mask, incross section and top view respectively.

FIG. 13 illustrates the fixture for three-dimensional ablation usingexcimer laser.

FIG. 14 illustrates the fixture for the ablation process after rotationof chip and with an additional height set tool.

FIGS. 15a and 15 b illustrate cross sections of two different chippositions in the fixture of FIG. 13 and 14 for the operation of theexcimer laser.

FIGS. 16 and 16b illustrate cross sections showing the adhesivepolyimide film and its application for stacking a multitude ofindividual chips into a cube.

FIG. 17 illustrates a cube with rerouting and solder material forsubsequent assembly onto a substrate.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 1, based on known technology, there is shown across section of a semiconductor chip 10, typically silicon, protectedby a dielectric protective overcoat 11, usually silicon nitride, and apatterned metallization 12 over the aluminum 13 of the circuit contactpads. Metallization 12 usually consists of a sequence of thin films,typically a refractory metal 12 a, such as chromium, in contact with thealuminum 13, followed by a solderable metal 12 b, such as copper, nickelor palladium. Finally, solder ball 14 is formed by reflowing theevaporated or plated solder alloy. After such attachment, solder ballsare often referred to as solder “bumps”. For fabricating this structure,several process flow variants, commonly referred to as “C-4”, have beendiscussed in the literature. They comprise at least ten process stepsand are thus expensive: Sputter Cr and Cu (or Ni or any of a wideselection of metals described in the literature); spin resist; bake;expose; develop; etch metal; remove resist; seed solder; evaporate orplate solder; reflow solder; flip chip attach.

FIGS. 2a and 2 b show a chip 20, made of silicon, gallium arsenide, orany other III-V or II-VI semiconductor material, having circuit contactpads 21 thereon (for example aluminum) wherein it is desired to rerouteelectrically conductive paths to said contact pads. The area between thecircuit contact pads is covered by protective, electrically insulatingovercoat 22. The rerouting is accomplished by initially sputtering,evaporating, or otherwise depositing a layer of electrically insulatingand temperature tolerant material, such as inorganic compounds likesilicon nitride, silicon dioxide, or silicon carbide, so that it coversthe side edges 23 of the chip 20, but not the circuit surface of chip20. This is illustrated in FIGS. 3a and 3 b. By way of explanation, eachchip, being a cuboid, exhibits six faces; the top face contains theintegrated circuit and is often referred to as the “circuit surface”;the bottom face; and four sides around the edges, often referred to asthe four “edge sides”; these edge sides are orthogonal to the top andthe bottom faces of the cuboid. The process described above results in alayer of uniform thickness, although not critical.

Two different methods of area protection during deposition of theelectrically insulating material are part of this invention: first,shielding of the entire circuit surface by positioning the chips withthe circuit surface down, and second, protecting the circuit contactpads by selective masking. In both variations, the insulating materialwill be preferentially deposited on the four edge sides of the chip.Both methods are discussed in more detail below.

Referring now to FIGS. 4a and 4 b, metal layer 24 is then deposited overthe entire circuit surface and the edge sides of the chip, following thecontours to make ohmic contact with the circuit metal pads 21. Accordingto the invention, metal layer 24 is preferably fabricated by depositingonto the circuit surface and the edge sides first a layer of refractorymetal 24 a, such as titanium or titanium-tungsten alloy, approximately40 to 700 nm thick (preferred thickness 50 nm), followed by a layer 24 bof platinum or platinum-rich alloy, approximately 200 to 800 nm thick(preferred thickness 500 nm). In order to obtain enhanced electricalconductivity, a layer of copper (50 nm to 1000 nm thick, preferredthickness 700 nm) may precede the deposition of the platinum. Otherrefractory metals include chromium, molybdenum, or tungsten. The layersare preferably deposited by a sputtering process, though any technique,such as vapor deposition or evaporation, can be used which will depositthe metals on the circuit surface and the edge sides.

Furthermore, since the metal films are so thin, any mechanical stressexerted in the semiconductor chip is minimized, as compared to theconsiderably thicker metallization films used in the known technology;less stress translates into avoidance of semiconductor bow and ofpeeling of metal, and thus results in higher production yield.

Since platinum has the advantage of very low dissolution rates in liquidlead-tin solders and does not form oxides, it is the preferred choice asthe undermetal for a wide variety of lead-tin solder mixtures, leadalloys or tin alloys, covering the wide range of technically usefulmelting or reflow temperatures from the eutectic to about 400° C.

Since both platinum layer 24 b and tungsten layer 24 a are thin, a laseris required as an ablation tool to pattern the layers. The invention canuse two methods. In one method, a mask is provided over the metallayers. The exposed metals are ablated by an excimer laser to provide aconductor pattern on the circuit surface of the chip as well as over oneor more of the edge sides of the chip, as illustrated in FIGS. 5a and 5b. The simplest embodiment of such patterning on a semiconductor circuitchip 30 is indicated by the cross section of FIG. 6, where therefractory metal layer 33 a and the platinum layer 33 b cover just thecircuit metal contact pads 31 (typically aluminum) and a smalldonut-shaped area around it. The area between the metal contact pads 31is protected by the protective overcoat 32. FIG. 6 also illustratesattaching prefabricated solder balls 34 to some contact pads before theyare processed through the reflow temperature cycle.

In another method, the laser is computer-controlled and thus able toproduce metal patterns of variable line widths, especially as small aschip-compatible feature sizes. With this capability, it is also possibleto generate some metal line widths wide enough to become groundpotential or power supply conductors which branch out into thinnerconnector lines in different parts of the pattern.

In either method, the employment of a laser eliminates chemical wasteand the use of hazardous chemicals such as aqua regia. Furthermore,redeposition of the removed material is avoided. Consequently, the largenumber of process steps needed for rerouting in the known technology isreduced by the invention to only four steps: Deposit titanium andplatinum; laser ablate; attach solder ball or print paste; flip chipattach.

FIG. 7 illustrates the attachment of prefabricated solder balls 34 toone contact pad 31 as well as to part of a rerouted connector 35reaching from the circuit surface to one edge side of the chip. Thesolder ball is shown before reflow. The fact that plating or evaporationof the solder material, as performed in the known technology, can bereplaced by the use of prefabricated balls, represents a significantprocess simplification and cost reduction. In addition, it is a majoradvantage of the present invention that the very low dissolution rate ofplatinum into high-temperature melting solders (for instance between 300and 400° C.) permits the use of two or more different solders fordifferent chips. In a sequential assembly, the chip with the highermelting solder can be attached to a substrate first and will remainthere fixed without reflow when a second chip with lower melting solderis attached later. Furthermore, the second chip can be reworked orreplaced without affecting the attachment of the first chip. Thisconvenient feature for easy repair may be repeated numerous timeswithout detrimental effect on the solder quality.

The principal features of the process described above for semiconductorchips can also be applied to substrates made of oxidized silicon,polymer, ceramic or other materials with an insulating surface so thatone or more levels of an interconnecting network of conducting paths andpads will be generated on the substrate. By way of example, FIG. 8illustrates an interconnect substrate 40, designed for the assembly ofone active semiconductor device and one passive element. Pads 41delineate the footprint of a semiconductor device, while pads 42delineate the footprint of a passive element, and pads 43 provideconnection to circuitry off the substrate 40. The fabrication of theconductor network follows the procedure described above and illustratedin FIGS. 4a, 4 b, 5 a, and 5 b. This means that the top surface of theconductor network consists of platinum or platinum-rich alloy.Consequently, prefabricated solder balls 44 may be applied directly tothe pads of the substrate, as indicated on two of the pads 41 intendedfor the assembly of the semiconductor device. It furthermore impliesthat the feature size of the interconnecting lines 45 may be as fine asthe one employed for interconnection in the semiconductor circuit.

After assembly of a circuit contact pad 52 of the chip 50 to a substrate51, a cross section through this single connecting junction isillustrated in FIG. 9. The chip contact pad 52, the titanium layer 53 aand the platinum layer 53 b, and the solder material 54 form an ohmicconnecting junction.

Assembling a chip, with rerouting connecting network prepared accordingto the invention, by one of its edge sides on a substrate, results in aproduct as shown in FIG. 10. In this configuration, electricalcharacterization and testing of the semiconductor chip can easily beperformed. Thereafter, the solder connection can be reopened byreflowing the solder connection, without detrimental side effects to thechip or the substrate. This procedure, based on the invention, thusrepresents a low-cost method of producing a “Known Good Die”—a productwhich can be achieved by known technology only in cost-intensiveprocedures.

As mentioned above, the process of depositing the insulating material onthe edge sides of the chip, while protecting the circuit surface of thechip, is an important part of this invention. To accomplish this goal byconventional methods, one would need photolithography with seven processsteps: Coat photoresist; bake photoresist; expose photoresist; developphotoresist; deposit dielectric; strip photoresist; and ash to clean.The process according to the invention consists of only one step, namelyto deposit the dielectric material. This simplified process is enabledby the apparatus illustrated in FIGS. 11a, 11 b, 12 a and 12 b.

Referring now to FIGS. 11a and 11 b, a chip 60 is placed on a support 61with its circuit surface down. A magnet 62 under the support exertsforce on a steel tool 63, which is placed across the inverted chip, thuspressing the chip flat against the support. Using vapor deposition asthe preferred process, insulating material, such as silicon nitride, isdeposited on the side edges of the chip, but substantially none on thecircuit surface of the chip. Vapor deposition not only permits reliablecoverage by insulating material of the edge sides, but also of thecorners of the chip where the edge sides meet the circuit surface atright angles. Other suitable processes include sputtering andevaporation. Preferably, the insulating material consists of siliconnitride; other suitable choices include inorganic materials such assilicon dioxide or silicon carbide.

For the process illustrated in FIGS. 11a and b, the steel tool 63 may beshaped as a simple bar or strip. However, in another modification ofthis invention, the chip may be positioned on the support with itscircuit surface up. In that case, the steel tool is configured to act asa mask. In addition to the edge sides, the deposition would then coverthose parts of the circuit surface which are not masked by the steeltool. As an example, FIG. 12b delineates the steel tool 64 in a shapeneeded for memory chips with center line positioning of the circuitmetal pads. In that case, the steel tool needs to mask these circuitmetal pads and some selected other areas of the chip; consequently, itmay have the shape of a center spine supporting fishbone-like structuresof side bars with widened ends.

It is important for cost-effective mass production to employ equipmentsuitable for batch processing of individual chips, preferably operatedby robots minimizing non-value-adding queue times.

One such apparatus which is an integral part of this invention isillustrated in FIGS. 13 and 14. The purpose of this apparatus is toprecisely position and hold a multitude of chips so that the pattern ofthe thin film metal conductors can be generated. Referring now to FIG.13, one chip 70 (out of a multitude of approximately 100) is shown on asupport 71, held by the pulling force of reduced air pressure (“vacuum”)supplied through numerous holes 71 a opened in support 71. Loading andunloading of the chips is preferably performed by a robot. The support71 is connected to a rod 72, held by bearings 73 in frame 74. Rod 72 canrotate around its axis for at least 90°. When rod 72 is positioned sothat one edge side 70 a of chip 70 faces upward, as shown in FIG. 14,the height set tool 75 is gently lowered, until it touches said chip andcomes to rest on pins 76. All other chips which may be adjacent to chip70 will be aligned simultaneously. As a result, all chips on support 71will orient their respective edge sides in one plane (with a precisionof approximately plus/minus 20 μm). The excimer laser for ablation willbe focussed onto this plane. Said focus plane has been given thereference designator 77 in FIGS. 15a and 15 b.

The excimer laser operates at high precision (focus considerably lessthan 25 μm) so that fine feature sizes of the thin rerouting metallayers can be produced, approaching the feature size of theinterconnecting metallization in the semiconductor circuit. As a result,the conductors for rerouting can be generated in fine feature size evenon the edge sides of the chips. Exploiting the fact that the laser iscomputer controlled, different widths of the metal lines can be obtainedfor different portions of a conductor. This capability enables variousline geometries in the rerouting pattern (“variable” feature size),allowing for instance the fabrication of wider electrical ground orpower supply lines before they branch off to a multitude of finer linewidths for connecting to the circuit metal pads.

By rotating rod 72 by 90°, the circuit surface of the chip 70 gets intothe focal plane 77 of the excimer laser, as indicated in FIG. 15b. Thisenables the laser again to fabricate the fine features sizes of thererouting metal pattern. It is an important characteristic of theequipment of this invention that it generates the same length referenced“L” in FIGS. 15a for the distance from the axis of rod 72 to the surface70 a of the edge side of chip 70, as it will generate the lengthreferenced “L” in FIG. 15b for the distance from the axis of rod 72 tothe surface 70 b of the circuit surface of chip 70. While the excimerlaser represents the preferred method to generate the fine feature sizepattern for the rerouting connectors of this invention, other suitablemethods include thick photoresist mask and lift-off, ion beam, andchemical etching techniques.

For stacking a multitude of chips (approximately ten or more) into athree-dimensional assembly or “cube”, this invention overcomes the primedifficulty of controlling the coefficient of thermal expansion (CTE)across the cube. This difficulty dominated known technology based onliquid adhesives (epoxy or polyimide). According to this invention, athree-layered dry film (thickness range 50 to 150 μm) is used,consisting of an upilex or polyimide center (approximately 20 to 80 μmthick), with almost polymerized polyimide or acrylic adhesives on bothsides (each layer approximately 20 to 40 μm thick). A cross section ofsuch sandwich film is illustrated in FIG. 16a, showing the upilex centerfilm 80 and the adhesive films 81. These materials are selected so thattheir combined CTE approximates CTE of silicon or other semiconductorsas much as possible. This goal is maintained even for the highprocessing temperatures of high-melting solders (up to about 400° C.).Furthermore, by appropriate selection of the thicknesses of layers 80and 81, desired electrical characteristics can be achieved for productparameters such as capacitve coupling and cross talk between conductorsof the chips or the cube.

In addition, the film sandwich is chemically clean, eliminates voidingand does not bleed out-features which contribute to high productreliability. Since the adhesives are almost polymerized, they need onlylittle time to finalize (“snap cure”, less than one minute, compared toabout two hours for conventional adhesives). Furthermore, since theinvention obsoletes adhesive dispensing and chip picking-and-placing,only minimal labor is needed for producing the cubes, resulting inreduced manufacturing costs. It is a major advantage of this inventionthat the sandwich film can be cut precisely in order to obtain the exactsize preform needed for manufacturing multi-chip cubes. As shown in FIG.16b, the adhesive sandwich preforms 82 act as spacers between thererouted chips 83, and can be assembled so that respective edge sides ofthe chips extend over the edge of the dielectric spacer, creating acastellated outline. This configuration enables easy attach of soldermaterial in the shape of prefabricated balls 84 to the extended edgesides 85 of the semiconductor circuit chips. In addition, the materialsof the sandwich preforms 82 have been selected so that they areinsensitive to the reflow temperatures of the solder bumps.

FIG. 17 illustrates a three-dimensional circuit assembly in theconfiguration of a cube 90, fabricated as described above, after it hasbeen soldered orthogonally onto the conductor pattern of a substrate 91,prepared according to the invention as described earlier. This methodpermits a high number of soldered contact points due to the thinness ofthe dielectric spacer and the fine feature size of the reroutedconductors on the edge side of the chips.

What is claimed is:
 1. A method for the fabrication of an integratedcircuit assembly comprising the steps of: providing semiconductorcircuit chip having a plurality of metal contact pads on the circuitsurface thereof; forming a plurality of patterned thin refractory metallayers connecting at least one of said pads with an edge side of saidchip; forming a thin film comprising platinum on a least one of saidrefractory metal layers, including an edge side portion; and connectinga solder ball to an edge side portion of said platinum—comprising film.2. A method a in claim 1 further comprising the step of orthogonallyattaching said assembly to a substrate having metal patterns thereon, byelectrically connecting said solder ball with one of said substratemetal patterns.
 3. A method according to claim 1 wherein said thinconnecting metal layers comprise platinum or platinum-rich alloy.
 4. Amethod according to claim 1 wherein said refractory metal layercomprises chromium, molybdenum, titanium, tungsten, or titanium-tungstenalloy.
 5. A method according to claim 1 wherein said solder bumpscomprise at least one alloy with a melting temperature compatible withmultiple reflow.
 6. A method according to claim 1 wherein said metallayers are deposited by sputtering.
 7. A method according to claim 1wherein said metal layers are patterned by laser ablation.
 8. A methodaccording to claim 1 wherein said solder material comprisesprefabricated balls.